Polyester films represented by polyethylene terephthalate films are widely used in various fields such as the magnetic recording and electric equipment fields because they have excellent mechanical, thermal and electrical properties.
Among the application fields of polyester films, there are magnetic recording tapes and magnetic recording disks, for which smoothness and good slipping properties of the surface, as well as good shaving properties at the time of application of magnetic material are required. In cases where the polyester film is used as video tapes, since the tapes are manufactured by slitting a film to a width of 1/2 inch or 8 mm and the shape of the cut face is one of the important factors which determine the quality of the tape, the films are also required to have a good slitting property. More particularly, magnetic tapes are generally manufactured by drawing out a polyester film rolled on a spool while simultaneously applying a magnetic material to one surface thereof, drying the film to remove the solvent, calendering the resulting film to plane the applied magnetic material to mirror-like state, winding the resulting film on a spool and then slitting the film to a prescribed width. Conventionally, in the slitting step, whiskers are formed at the cut faces, which causes degradation of the performance of the products. For example, bad slitting properties may cause drop out or clogging of the head of the video tape recorder when the film is used as a magnetic tape.
In the calender step, the polyester films to which the magnetic material has been applied are passed between a rotating elastic roll and a rotating heated metal roll under a high nipping pressure, such that, in general, the surface to which the magnetic material is applied contacts the metal roll and the opposite polyester surface contacts the elastic roll. In this step, a plurality of alternately arranged elastic rolls and heated metal rolls are used. When the polyester film passes between the rolls, shaving of the film is performed because of the high shearing force exerted to the surface of the polyester film contacting the elastic roll. The higher the speed of the calender step, the more the abradings generated. The abradings produced in the calender step may be re-attached to the surface of the polyester film and then may be transferred to the surface to which the magnetic material is applied when the tape is rolled to cause drop-out which is one of the serious defects of the electro-magnetic conversion characteristics of magnetic tapes. Thus, it is important to reduce the production of the abradings. In other words, it is important to improve the shaving property of the film.
As to the improvement of the slipping property, for example, Japanese Patent Publication (Kokoku) No. 20496/80 discloses a polyester film containing both of the internal particles and inert incorporated particles. However, the polyester film disclosed in this reference does not have a satisfactory slitting property.
As a polyester film with an improved slitting property, for example, Japanese Patent Disclosure (Kokai) No. 11179/79 discloses a polyester film with a specific Young's modulus of elasticity in the machine direction and the transverse direction. Japanese Patent Disclosure (Kokai) No. 159619/82 discloses a polyester film with an improved slitting property, which has a specific relationship between the refractive index in the direction of thickness of the film and the intrinsic viscosity thereof. Further, Japanese Patent Disclosure (Kokai) No. 162126/82 discloses a polyester film with an improved slitting property, which has a specific relationship among the refractive index in the direction of thickness of the film, the refractive index of the film in the machine direction and transverse direction, and the intrinsic viscosity of the film. However, the polyester films disclosed in these references do not have a satisfactory slipping property.
As a polyester film with an improved shaving property, for example, Japanese Patent Disclosure (Kokai) No. 254328/86 discloses a polyester film in which the shaving property is improved by incorporating dispersed silicon oxide secondary particles composed of primary particles, which have a large inner surface area. However, the polyester film disclosed in this reference does not have a satisfactory slitting property.
Thus, heretofore, there is no polyester film which has both a good slitting property and good slipping property. Further, heretofore, there is no polyester film which has a good slitting property, good slipping property and good shaving property.